Technical Field
A method is disclosed for controlling a linear or rotary multi-actuator drive device having a stationary part and a movable part, wherein relative movement between the stationary and the movable parts is generated via actuators having limited strokes, which are in continuous frictional contact with the movable part either directly or via a force-transmitting mechanism.
Description of the Related Art
Several drive technologies that allow large adjustment paths, despite the actuators having short strokes, are known from the prior art. These technologies can be subdivided into various different variants. Different types of actuators having limited strokes can be used. Piezoelectric and electrostrictive actuators are particularly suitable. Both linear and rotary movements can be generated by means of such drive devices.
The precursors of multi-actuator drives are inertia drives, specifically in this case the stick-slip drives known from D. W. Pohl, “Dynamic piezoelectric translation devices”, in Review of Scientific Instruments, Vol. 58 (1), January 1987, pages 54-57. In such drives, a piezoelectric actuator is provided to which a periodic, sawtooth-like signal is applied, and which produces an acceleration relative to a displaceably mounted runner, frictionally connected to the actuator. When the acceleration of the piezoelectric actuator is low, the runner follows the actuator due to frictional engagement. When the acceleration of the piezoelectric actuator is high, in contrast, the runner slips relative to the actuator as soon as the inertial force of the runner is greater than the frictional force between the runner and the actuator. When several steps are performed, it is also possible for macroscopic movements to be realized. Inertia drives are a mechanically simple way of positioning objects over large distances and with high movement resolution. However, a disadvantage of inertia drives is that braking, standstill or even backwards motion of the movable components can occur again and again during the slip phase. This behavior couples-in vibrations, which are disruptive for precision positioning tasks.
The principle of an inertia drive 100, in which an actuator D is fixed to one side of an immobile mass F can be seen from the drawings in FIGS. 1A and 1B. Body E is in frictional contact with actuator D. When actuator D is slightly accelerated by applying a slowly changing voltage as shown in FIG. 1B, body E is moved with it. When actuator D accelerates strongly, the inertial force of body E is greater than the static friction, thus generating relative movement between body E and actuator D. Early multi-actuator drives were similar to such an inertia drive and differ initially in that two or more actuators are used.
FIG. 2A shows a basic structure of a prior art linear multi-actuator drive 102, and FIG. 2B shows the basic structure of a rotary multi-actuator drive 104 known from the prior art. Such multi-actuator drives comprise at least two actuators 11, 12, 1n, which can be driven individually or in groups by a respective control signal to perform a limited stroke. If piezoactuators are used, the stroke typically ranges up to a number of microns. The actuators each have a point of friction 2 which is in frictional contact with a runner 3. The actuators are also fixedly connected to a base 4. The points of friction need not necessarily consist of one plate only, as shown in the Figures, but may also be mechanically complex constructions that perform several different functions. For example, it is possible that this component is used to mechanically tension the actuator as well, as is common in the case of piezoceramics, for example.
The structure of multi-actuator drives can also be inverted, of course, with the runner 3 being the stationary component and the base 4 being the movable component. The drive principle is still the same in either case.
FIG. 3 shows a set of normal time-voltage waveforms 106 for controlling a plurality of actuators, which are controlled with a timing offset 108 by means of sawtooth voltages 110, similarly to a classic inertia drive, and the typical resultant movement 112 for the runner 3.
For specific realizations, reference is made to the publication by Jean Marc Berguet entitled, “Actionneurs ‘Stick and Slip’ pour Micro-Manipulators,” (EPFL, 1998), which describes, for multi-actuator drives, based on the teaching in EP 0750356 A1, with two, three, and four actuators per drive, that the variations in speed and the vibrations typical for inertia drives are less in a multi-actuator drive. An interesting aspect of the solution is that the multi-actuator drive utilizes the inertial forces of the movable component, due to the strong acceleration of the piezoactuators, so that low-vibration movement in comparison with simple inertia drives can already be achieved with just two actuators. The technique disclosed in EP 0750356 A1 is also described in US 2010/0314970 A1 and US 2008/0191583 A1.
In such drives also, due to their principle of operation, the runner may make an undesired backwards movement 114 or stop or be braked when an actuator transitions to the slip phase. FIG. 3 visualizes the case of slight backwards movement.
A special form of multi-actuator drive is described in WO 93/19494, where the individual friction surfaces are progressively made to slip as a result of rapid deformation of the piezoceramics. The friction surfaces are subsequently deflected together in one direction by applying an identical voltage ramp. When this synchronous deflection occurs, the runner does not slip relative to the friction surfaces, but is entrained instead. However, it is disadvantageous that the runner is exposed to strong vibrations due to the constant changes in acceleration.
According to DE 10 2009 013 849 A1, a drive based on piezo tubes and having a plurality of friction surfaces is controlled with a timing offset in order to effect a movement. The control signals are chosen so that a plurality of actuators drive a runner, in order to progressively retract the actuators such that the runner is held in a position by the plurality of unmoved friction surfaces during refraction, i.e., it stands still. In this drive, phases of motion and standstill repeatedly occur for the runner, with the result that vibrations continue to occur and smooth motion is not possible.
“Inchworm drives” are another class of multi-actuator drives. These are drives in which a runner to be moved is alternately clutched by actuators, the distance between the two sets of clutching actuators being varied by a further actuator. A runner can also be moved over large distances by controlling the actuators in an appropriate cyclical manner. This class of actuators generally causes disruptive vibrations, in that high-frequency movements acting orthogonally to the direction of motion are coupled-in by the clutching operations.
In prior art multi-actuator drives, undesired vibrations and deviations of the actual position of the runner from a target position occur. This effect is particularly great when one or more frictional contacts transition from the stick phase to the slip phase.